Minimum divertor leg length for detached divertor operation compatible with a high performance core plasma

Poloidal gradients in the outer leg of DIII-D detached divertor discharges are characterized for minimum leg length for high divertor dissipation without perturbative cooling of the X-point region. Measurements from the divertor Thomson scattering (DTS) diagnostic indicate poloidal electron temperature gradients of up to 200 eV/m through the dissipative region of a detached divertor leg. The gradients are consistent with convective-dominated transport and dissipation due to intrinsic and seeded low-Z impurities. The T_e poloidal scale length is estimated as L_Te≈v_polq_eT_e/L_zf_zn_e where v_pol is the effective plasma poloidal velocity from parallel convection and E×B poloidal flow, L_z is the radiative loss parameter, and f_z is the radiating impurity fraction. For DIII-D high performance scenarios with ≥ 10 MW of injected power and a goal of high divertor dissipation while maintaining the X-point region at T_e ≥ 80 eV, a minimum divertor leg length of ≥ 40 cm is indicated. This is a factor of 2-3 longer than the typical DIII-D configuration and would present a target for future divertor designs. For future tokamaks the scaling appears to be favorable with larger size, higher field and higher density, but will also depend on the scaling of radial transport and plasma drifts.